The present invention relates to ion sources and ion implantation systems for generating and implanting ions into a substrate, and more particularly, ion implantation systems that facilitate implantation of low doses of ions into a substrate.
Ion implantation has become a standard, commercially accepted technique for introducing conductivity-altering dopants into a substrate, such as a semiconductor wafer or thin film deposition on a glass substrate, in a controlled and rapid manner. Conventional ion implantation systems include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. This beam is directed at the surface of the substrate. Typically, the energetic ions of the ion beam penetrate into the bulk of the substrate and are embedded into the crystalline lattice of the material to form a region of desired conductivity. This ion implantation process is typically performed in a high vacuum, gas-tight process chamber which encases a workpiece handling assembly, a workpiece support assembly, and the ion source. This high vacuum environment prevents dispersion of the ion beam by collisions with gas molecules and also minimizes the risk of contamination of the substrate by airborne particulates.
In the ion source, a dopant gas, such as phosphine or boron, and a diluent gas, which is typically nitrogen or hydrogen, are ionized by an energized cathode to form an plasma. This plasma is accelerated by suitable electrodes through the ion source to form an ion beam, which is then introduced into the implantation chamber of the ion implantation system. The ions present in the ion beam impinge upon the substrate and are implanted therein.
A drawback of conventional ion sources and implantation systems is that the dopant gas typically coats the chamber walls of the ion source with a residue. The diluent gas when introduced into the ion source plasma chamber reacts with the residue coating and creates impurities increasing the effective concentration of dopant gas in the dilutant gas. This uncontrolled release of dopant into the plasma prevents control of the low dose implants. One method of avoiding the introduction of impurities into the implantation chamber would be to clean the residue from the walls of the ion source after each use. This methodology, however, is impractical since it results in a significant loss of service time of the ion source, with a corresponding decrease in throughput in the ion implantation system.
Although the impurities created by the reaction between the dopant residue and the diluent gas can typically be tolerated in high dose applications, there is an ever increasing demand for ion sources that implant low doses of ions into substrates. In such applications, rigid control over the constituency of the beam current and ions are necessary. The presence of impurities in the ion beam makes it difficult to control the ion dose being implanted in the substrate.
Hence, there still exists a need in the art for an ion source and implantation system that facilitates the implantation of low doses of ions into a substrate. In particular, an ion source that can be utilized regardless of the type of prior use, known generally as "source history," and without requiring frequent cleanings to remove residue would represent a major improvement in the art.
The invention will next be described in connection with certain preferred embodiments. However, it should be clear that various changes and modifications can be made by those skilled in the art without departing from the spirit and scope of the invention. For example, various systems employing various system structural components that utilize the preferred practice of the invention can be employed to use a non-reactive diluent gas.